1. Field of the Invention
This invention relates generally to a method for controlling sludge temperature in the autothermal aerobic digestion of such sludge.
2. Description of the Prior Art
U.S. Patent Application Ser. No. 027,801 filed Apr. 6, 1979 in the names of M. S. Gould et al describes a process for digestion of sludge wherein the sludge is oxygenated with an aeration feed gas containing at least 20% oxygen (by volume) in a first digestion zone at temperature of from 35.degree. to 75.degree. C. to partially reduce its biodegradable volatile solids content. The resulting partially stabilized sludge is then anaerobically digested in a covered second digestion zone at temperature of from 25.degree. to 60.degree. C. to yield a further stabilized sludge residue. In this process, the aerobic digestion in the first digestion zone is conducted for sludge retention time of 4 to 48 hours or, as alternatively specified, so as to reduce the volatile solids content of the sludge introduced to the first digestion zone by from 5 to 20%.
Within the above-mentioned broad ranges of sludge temperature and retention time, several embodiments of the foregoing process are possible. In one particularly advantageous embodiment, an autothermal aerobic digester is positioned in front of an anaerobic digester. Feed sludge having a relatively high total suspended solids content, e.g., 30,000 mg/l, is introduced into the aerobic digestion zone along with an oxygen-containing aeration feed gas, preferably containing at least 50% oxygen (by volume). The aeration feed gas preferably contains at least 50% oxygen (by volume), inasmuch as heat losses associated with an oxygen-containing gas of a lower oxygen concentration tend to become sufficiently large as to hinder autothermal thermophilic operation. Such high oxygen concentration gas is also preferably employed in order to secure an increased rate of oxygen mass transfer to the sludge during aerobic digestion to in turn provide an intensified aerobic digestive action.
In the aerobic digestion zone, the oxygen-containing aeration feed gas is contacted with the sludge, with aerobic digestion being continued for sufficient duration to satisfy one of the sludge retention time limitations (sludge retention time or sludge volatile solids reduction) discussed earlier herein. By utilizing a properly insulated, covered aerobic digestion zone, by maintaining the total solids content of the sludge in the aerobic digestion zone at a high level, and by utilizing a high oxygen content gas, one is able to maintain thermophilic operating conditions, i.e., a temperature of between 45.degree. and 75.degree. C., in the aerobic digestion zone autothermally.
Thermophilic operation of the aerobic digestion zone is preferred for several reasons. First, the rate of digestion in the aerobic digestion zone is directly related to temperature, with higher temperature generally corresponding to higher rates of digestion. As a result, the retention time necessary to achieve a given level of aerobic digestion will be shorter at higher temperatures. Second, by operating at thermophilic temperatures one is better able to maintain the subsequent anaerobic digester in the process within its optimum temperature range regardless of ambient temperature variations. Finally, by operating at thermophilic temperatures, and more specifically by operating at temperatures above about 50.degree. C., one is able to ensure that the partially stabilized sludge discharged from the aerobic digestion zone is completely pasteurized. Pasteurization is a reduction of the concentration of pathogenic organisms in the sludge.
Following initial aerobic digestive treatment, partially stabilized sludge is passed from the aerobic digestion zone to an anaerobic digestion zone. In the anaerobic digestion zone, the sludge contents of the zone are continuously mixed, thereby creating a large zone of active decomposition and significantly increasing the rate of the stabilization reactions. A further stabilized sludge and a methane-containing digester gas produced in the anaerobic digestion are separately discharged from the anaerobic digestion zone.
The above-described digestion process has proven to be extremely stable in operation and highly resistant to process upsets due to fluctuations in process operating conditions, such as are common to conventional anaerobic digestion processes. Moreover, by utilizing biologically generated heat developed in the aerobic digestion zone to thermally stabilize the anaerobic digestion zone, more of the heat energy produced by the anaerobic digestion (i.e., in the form of combustible digester gas) is available for other uses. Nevertheless, this aerobic/anaerobic digestion process does have one significant drawback in commercial use. Due to variations, both periodic and non-periodic, in the strength (biodegradability), solids concentration and feed rate of the sludge introduced into the aerobic digestion zone, the temperature in such digestion zone may vary significantly. This temperature variation is evident even though the changes in strength, solids concentration and feed rate of the influent sludge to the aerobic zone are within the broad limits of operability disclosed in U.S. Ser. No. 027,801 for the aerobic/anaerobic sludge digestion process. Such changes in the character and flow of the influent sludge to the aerobic digestion zone may thus adversely effect the operation of the plant and the associated efficiency of sludge digestion treatment.
As an example of the foregoing, a sudden decrease in the amount of the sludge fed to the aerobic digestion zone gives rise to an increased sludge retention time in the aerobic digestion zone, the retention time being inversely related to the volumetric flow rate of sludge fed to the zone. The sudden decrease in the amount of sludge fed to the aerobic digestion zone will result in the temperature rising above the value selected for optimum operation at steady-state conditions and design values. Such undesirably high temperature condition gives rise to an excessive usage level of oxygen, as based on a unit mass of sludge in the aerobic digestion zone. Apart from increasing the operating costs for the digestion system due to increased oxygen consumption an excessive oxygen utilization level also results in a reduction of the quantity of volatile solids in the sludge that is fed to the subsequent anaerobic digestion zone. Such volatile solids reduction in turn reduces the amount of methane gas that can be produced by the downstream anaerobic step. On the other hand, an excessively low temperature in the aerobic digestion zone, such as may occur due to a sudden increase in the feed rate of sludge introduced to the aerobic digestion zone, may result in inadequate pasteurization of sludge, so that pathogenic organisms are passed from the aerobic digestion zone into the subsequent anaerobic digestion zone and ultimately pass out of the treatment system with the further stabilized sludge product from the anaerobic digester.
Finally, temperature fluctuations in the aerobic digestion zone may, if of sufficient magnitude, cause temperature fluctuations in the anaerobic digestion zone, such as could lead to an upset in the digestive process conducted in the latter zone.
Accordingly, it is an object of the present invention to provide a method for controlling temperature of sludge being treated in an autothermal aerobic digestion zone.
It is another object of the invention to provide a control method which modulates the consumption of oxygen-containing aeration feed gas in a highly efficient manner, as well as limiting the extent of digestion in an autothermal aerobic digestion zone disposed upstream of an anaerobic digestion zone, in such manner as to ensure that methane production in the downstream anearobic zone is maximized.
Other objects and advantages of the invention will be apparent by way of ensuing disclosure and the appended claims.